Aerosol measurements characterize the size, concentration and composition of particles suspended in the atmosphere. Measuring the particle size distribution provides the concentration of particles as a function of size. Atmospheric particles influence climate change, radiative transfer, visibility, and air quality. Measurements of the concentration and sizes of particles present in the atmosphere allow quantification of pollutant effects and monitoring particulate growth.
Aerosol instruments can be used for carrying out high-resolution, in-situ aerosol measurements from aircraft and ships. These measurements can probe the spatial and temporal variability of the tropospheric aerosol. A striking example of the effect of spatial variability of aerosol characteristics on cloud properties is provided by ship tracks, first observed from satellites, and later from aircraft measurements. Ship tracks provide a dramatic example of the ability of aerosols to alter the resulting cloud characteristics; measuring this microphysical evolution by means of in-situ measurements was an important goal of the Monterey Area Ship Track (MAST) Experiment. These spatially well-defined perturbations of the aerosol concentration and composition and of cloud properties provide an opportunity to study the broader question of the impact of anthropogenic emissions on cloud properties.
Theoretical studies have predicted that both marine and anthropogenically-influenced tropospheric aerosols should vary diurnally as a result of photochemical reactions resulting in secondary new particle formation and aerosol growth. Such work suggests that the aerosol size distribution will evolve during the day through a series of characteristic size distributions indicative of periods of nucleation and condensation. Providing in-situ evidence for such direct dependence of aerosol properties on other atmospheric variables suggests studies of marine boundary layer and free tropospheric aerosol with aircraft instrumented to measure size distributions quickly and automatically.
Airborne measurements of submicron aerosol size distributions at a frequency capable of resolving the differences, for example, between the cloud line features and surrounding clouds are desirable in order to characterize small-scale or ephemeral features in the atmospheric aerosol. Many of the commercially-available submicron aerosol classification and counter designs are not suited for this application partially because of the long sampling times required to characterize the submicron size distribution extending over two or more decades in particle diameter.
One type of the prior-art airborne aerosol instruments use optical particle counters aboard the aircraft. These instruments provide valuable insight into the variation of aerosol with altitude, and the character of aerosol in and above the clouds. One such system was described by Radke et al. for obtaining size distribution information with an optical particle counter (OPC) for particles greater than 0.1 mm diameter in "Direct and remote sensing observations of the effects of ships on clouds", Science, Vol. 246, pp. 1146-1149, 1989. Clarke et al. introduced the Thermo-Optical Aerosol Detector (TOAD) to characterize both the dry aerosol distribution and its volatility in 1991 ("A thermo-optic technique for in-situ analysis of size-resolved aerosol physicochemistry", Atmos. Env., Vol. 25A, pp. 635-644). Hegg et al. and Clarke extended the effective size range of aerosol measurement using mobility-classification to below 20 nm diameter. Detailed descriptions of their work can be found in "Aerosol size distributions in the cloudy atmospheric boundary layer of the North Atlantic Ocean", J. Geophys. Res., Vol. 98, pp. 8841-8846, 1993 and "Airborne measurements of aerosol properties in clean and polluted air masses during ASTEX", EOS Proceedings of the 1993 AGU Spring Meeting, Apr. 20 of 1993.
Several constraints are inherent to aircraft-based submicron aerosol measurement, including limitations on size, weight, and power as well as the necessity for making fast measurements while adjusting rapidly for changing pressure, temperature, and humidity conditions. The need for rapid measurements derives from the aircraft's speed relative to the spatial scale of changes in aerosol properties. The spatial resolution possible with an airborne instrument is determined both by the speed of the instrument and the speed of the aircraft. Conventional differential mobility analysis requires a sampling period of about 10 min. If continuous sampling methods were employed, the resulting size distribution would represent, for example, at a speed of 100 m/s, a composite distribution of sized aerosol concentrations for a 60-km flight leg. Since air mass characteristics can change drastically over 60 km, several prior-art systems employed a grab sampling approach in which air is drawn into a holding chamber and stored while a single measurement is processed. Radke et al. employed a 90 l steel cylindrical chamber with a floating piston filled by ram pressure to store the aerosol for size classification, and were thus able to store a sample collected in 5 seconds (see, the above referenced publication in 1989). Hegg et al. also employed a large (about 2.5 m.sup.3) polyethylene bag for analysis over a ten minute period of size classification (see, the above referenced publication in 1993).
The approach of grab sampling has successfully provided in-flight snapshots of aerosol in air masses, which have been coupled with continuous condensation nuclei (CN) measurements to determine the aerosol's spatial variability. Measurement speed still limits both the frequency with which complete distributions can be acquired and the instrument's lower detection limit. Diffusional deposition of aerosol particles on the walls of a sampling vessel can reduce the number concentrations dramatically for long counting times. Consequently, the chamber's volume must be chosen such that particle losses during sampling and analysis are minimized. Particle losses in a chamber are also exacerbated by electrostatic enhancement of charged particles on the chamber walls. Hegg et al. measured ultra fine particles during a 10-min. sample measurement protocol by employing a 2.5-m.sup.3 chamber.
To size particles smaller than 0.1 .mu.m in diameter, a differential mobility analysis is usually employed. This technique is described in detail by Knutson and Whitby, in "aerosol classification by electrical mobility" in J. Aerosol Sci., vol. 6, p. 453, 1975. A differential mobility analyzer separates charged particles according to their migration velocities in an applied electric field. Differential mobility analysis is accomplished by introducing a small aerosol flow near one electrode of a two-electrode apparatus, with a larger particle-free sheath flow separating that aerosol from the second electrode. An electrical potential drives particles of appropriate polarity across the sheath flow toward the opposite electrode. At a location downstream from the aerosol inlet, small classified aerosol sample flow is extracted, which the remaining flow is discharged to an exhaust. Only particles that migrate within a narrow range of velocities are included in the classified aerosol sample flow. Particles with higher migration velocities deposit on the counter electrode while those with lower migration velocities are discharged with the exhaust flow. In measurements of differential mobility size distribution, the classified aerosol particles are transported to a detector for counting. Because the particles of interest are too small to be efficiently detected optically, they are commonly grown by vapor condensation in a detector known as a condensation nucleus counter.
The particle size is inferred from the migration velocity based on the relationship between particle size and the electrical mobility of the particles. This is described by Flagan and Seinfeld in "Fundamentals of Air Pollution Engineering", Prentice-Hall, 1988. The electrical mobility, Z, is defined as the ratio of the migration velocity U.sub.m to the strength of the applied electrical field, E, ##EQU1## For spherical particles carrying V electrical charges, the mobility Z can be written as ##EQU2## where .mu. is the gas viscosity, D.sub.p is the particle diameter, C.sub.c is an empirically-determined slip correction factor that accounts for noncontinuous aerodynamic effects that become important where the particle diameter is comparable to or smaller than the mean-free-path .lambda. of the gas molecules, and e is the elementary unit of change. A commonly employed form for this slip correction factor is ##EQU3## where ##EQU4## is the Knudsen number. Under typical operating conditions, only a fraction of the particles are charged, and a majority of those charged particles carry one charge, i.e., V=1. Most mobility classifications are performed with positively charged particles.
The migration velocity required for a particle to be transmitted from the aerosol inlet flow to the classified aerosol outlet flow of the differential mobility analyzers depends on the geometry of the classifier and on the four flow rates, i.e., an input sample flow rate, an input sheath flow rate, an output sample flow rate, and an output excess flow rate. The size of the particles to be classified is selected by adjusting the voltage such that particles with the mobility of particles of the desired size will migrate at the velocity required for transmission. The size distribution of the aerosol is determined by making measurements of the concentrations at a number of sizes spanning the size range of interest.
Differential mobility analysis has traditionally been performed by making a sequence of measurements at different electric field strengths, i.e., at different voltages applied across the two electrodes of the classifier. Although this method is effective, it is slow, requiring times ranging from several minutes to more than an hour to measure a size distribution depending on the size range that is probed and the resolution that is sought. Wang and Flagan accelerated the measurements dramatically by exponentially ramping the voltage and counting the particles continuously, thereby eliminating the delays between successive measurements that are needed to ensure representative data at each mobility with the stepping-mode of differential mobility analysis. A complete size distribution can be measured in less than one minute with this accelerated scanning-mode of differential mobility analysis. Wang and Flagan disclosed this in "Scanning electrical mobility spectrometer" in Aerosol Sci. Technol., 13, pp. 230-240, 1990.
The speed with which aerosol size distributions may be characterized is limited by the time required to obtain significant particle counts for each size channel, which, for a given ambient concentration, is a function of the counting statistics and efficiency of the detector and the flow profile in the measurement system. In single-particle counting operation, the counting statistics of condensation nucleus counters (CNCs) are governed by the number of particles that can be counted in a specified time interval. For a stream of air containing N.sub.i particles (cm.sup.-3) with flow rate Q.sub.s at the detector, and detection efficiency s(D.sub.p, V) for fraction p(D.sub.p, V) of particles of diameter D.sub.p carrying V charges, the measured signal S.sub.i for channel i is proportional to the product, N.sub.i Q.sub.s s(D.sub.p, V s(D.sub.p, V).
Among the commercially available counters, the TSI 3025 condensation nucleus counter has one of the highest detection efficiency for ultra fine (less than 10-nm diameter) particles. However, in order to obtain uniform saturation the sample flow is surrounded by a sheath flow that dilutes the flow to the counter of 0.3 liter per minute by a factor of 10, so that Q.sub.s is 0.03 liter per minute. This was described by Stolzenburg and McMurry in "An ultrafine aerosol condensation nucleus counter" in Aerosol Sci. Technol., Vol. 14, pp. 48-65, 1991. Other condensation nucleus counters, in particular TSI models 3010 and 3022, have detector flow rates of 1 liter per minute, but have 50% detection efficiency cutoffs of 10 nm and 8 nm, respectively, such that size distributions may not be extended to the ultra fine range.
Airborne measurements of particle size distributions by differential mobility analysis are further complicated by pressure variations that accompany altitude changes. Mobility classification requires precise control of several coupled flow rates. Continuous measurements require immediate and accurate adjustment of those flows in response to changes caused by pressure variations. As discussed previously, the above-mentioned prior-art systems and methods for airborne particulate growth are manual-intensive, spatially and temporally resolution-limited. In addition, the signal to noise ratios in clean air are low.
The present invention discloses components and a system design for aerosol measurements with significantly improved spatial and temporal resolution, automated flow control, and high counting efficiency at particle sizes less than 0.5 mm. A preferred embodiment, the Automated Mobility-Classified-Aerosol Detector (AMCAD), has an alternating dual-bag sampler, a particle charger, an improved differential mobility analyzer (DMA), and a condensation nucleus counter (CNC). The implementation of automated feed back control of flow rates allows the preferred embodiment of the present invention to achieve high-resolution and high precision measurements under changing pressures. The AMCAD also controls the temperatures of the saturator and the condenser in the condensation nucleus counter to achieve consistent high counting efficiency as the temperature of the incoming aerosol sample changes. The adverse effects associated with the humidity level of the aerosol sample are reduced by desiccating the dilution flow that mixes with the aerosol sample flow at the entrance of the condensation nucleus counter.
The advantages, sophistication, and significance of the present invention will be more apparent in light of the following detailed description of the preferred embodiment thereof, as illustrated in the accompanying drawings.